1. Field of the Art
The present invention relates to a control apparatus for conducting variable speed control of an induction machine through vector control and more particularly, to a control apparatus for an induction machine comprising means for calculating a magnetization current command in accordance with a secondary flux command, means for calculating a torque current command in accordance with a torque command, means for calculating a slip frequency command in accordance with the secondary flux command and the torque command, means for generating a primary current command in accordance with the amplitude corresponding to that of vector sum of the magnetization current command and the torque current command and in accordance with the phase corresponding to that of the primary current command, and means for controlling the primary current of the induction machine in accordance with the primary current command.
2. Prior Art
Power converters such as inverters and cycloconverters can now be easily realized due to remarkable developments in the field of power semiconductor devices. Thus, induction motors heretofore generally used as constant speed motors have now been extensively applied to variable speed motors.
As the variable speed control methods for induction motors, the vector control method is frequently adopted due to its excellent response characteristics. In particular a flux detection type vector control method and a slip frequency type vector control method are known. The former method uses the vector quantity of detected secondary flux as a control signal for a primary current, and the latter method controls the primary current by a vector flux calculated in accordance with motor constants.
For a conventional slip frequency type vector control apparatus, it is necessary to calculate a magnetization current component command i.sub.1R *, a torque current component command i.sub.1I * and a slip frequency command .omega..sub.S *. However, the secondary resistance R.sub.2 of an induction motor is directly concerned with the calculation. As the temperature of the rotor rises, the secondary resistance R.sub.2 increases so that a difference becomes present between the calculated value and the actual value. Thus, the main flux varies and the calculation result of the slip frequency command .omega..sub.S * is influenced greatly so that the proper vector control characteristics cannot be maintained.
The influence of the secondary resistance is not associated with flux detection type vector control apparatus. However, there is a disadvantage that a secondary leakage inductance error is present since the calculated result is the secondary flux neglecting the secondary leakage inductance, i.e., only the gap inductance. In spite of such an error, the flux detection type vector control apparatus has the following advantage: even if the constants of the primary and secondary circuits of an induction motor vary, the voltage and current of each phase to be input to the flux calculator change so that the flux calculation result changes correspondingly, and the vector control characteristics are less degraded even if the circuit parameters vary.
From the point of view of practical circuit arrangement there are many problems regarding the precision and resolution of a flux detector, and regarding low calculation precision of a flux calculator due to voltage distortion at low speed operation which makes it difficult to obtain a sufficient starting torque.